Looking at John's suggestion I could perhaps be persuaded it is a very dark Sticto 12-pustulatus. Where was it found - running water or still waters, S-12 is more of a running water species. Certainly not Hydroporus palustris.

John,
I have no literature on Coleoptera. All I have M. Chinery's "Insects of Britain and Western Euroupe, totally dependent on the inernet, I am.
Please don't ask me to explain. There is one subject I can be explicit about, that being all the help and guidance I get from good folk such as yourself.